The basic operation and structure of trunking communication systems and cellular telephone communication systems (cellular communication systems) are known. Trunking communication systems typically comprise a communication channel controller, a limited number of repeaters that transceive information via communication channels, and a plurality of communication units which may be mobile vehicle radios and/or portable radios. Of the communication channels, one is typically chosen to be a control channel. The control channel typically transceives operational information between the communication channel controller and the plurality of communication units such that, for example, the plurality of communication units can access the communication channels. Typically, the trunking communication system has a relatively large geographic coverage area, depending on the environment that the trunking communication system is located, the coverage area may be approximately 35 miles in diameter.
A cellular communication system typically comprises a mobile telephone switching office (MTSO), a plurality of cells, a limited number of communication channels, and a plurality of communication units which may be cellular telephones. Each of the plurality of cells comprises some of the limited number of communication channels, wherein one of the communication channels is designated as the control channel for that cell. The control channel transceives operational information between the plurality of communication units within the cell and the MTSO such that the communication units can place telephone calls via a phone system. Typically, the coverage area of each cell is relatively small in comparison with that of a typical trunking system. For example, a typical cell coverage area is approximately two miles in diameter. Because an individual cell coverage area is relatively small, the communication channels may be reused, at least once, within a geographic region of approximately the same size as a trunking communication system.
In many instances, it is desirable for an operator of a console within the trunking communication system to locate a particular communication unit. Presently, automatic vehicle location (AVL) systems exist. To operate in these systems, communication units are typically equipped with at least a second transmitter. The second transmitter, upon receipt of a prompt, transmits, usually via satellite, a message that can be decoded to determine the communication unit's location. The AVL system accurately locates a communication unit, but requires an additional transmitter in each communication unit and a subscription to the satellite, both adding cost to the communication unit and the system.
Another method is the Coast Guard Loran-C system, or similar system, which comprises three beacon signals. A communication unit receives all three signals, and based upon the different times the signals are received, an exact location of the communication unit can be determined. This method also requires that a communication unit have an additional receiver to receive the beacon signals. Yet another location method is available in trunking communication system networks. A communication unit within the trunking communication system network can be located within the nearest trunking communication system. However, in many applications, the accuracy of the Loran-C location system is not need, yet the location system of a trunking communication system network is not accurate enough. Therefore a need exists for a relatively accurate communication unit location method that does not require the communication unit to include an additional receiver, a subscription to a satellite, or complexed location decompilers.